Generally in this type of speed change mechanism, a speed change program (speed change line) indicating the relationship between the speed of the automobile and the opening degree of the throttle valve is established in advance, and based on such a program, the pertinent speed change step is selected correspondingly with the speed of the automobile and the opening degree of the throttle valve. Therefore, in an automobile using this type of speed change mechanism, the running characteristics of the automobile are determined by the nature of the speed change program. Furthermore, an automatic speed change mechanism is also known in which multiple programs consisting of a program having superior fuel economy and a program having superior acceleration features are provided so that the driver can select the desired program through manipulation of manual switches.
However, if the program having the superior accelerating feature is chosen, the fuel economy is aggravated, while, if the program having the superior fuel economy is chosen, the accelerating characteristic is neglected. Furthermore, if the number of passengers varies, the load of the automobile is correspondingly varied, resulting in the variation of the accelerating characteristic. For example, even if a speed change program having relatively good fuel economy is chosen, if the load is very small, an adequate acceleration can be obtained, while if the load is very large, the acceleration characteristic is aggravated, thereby creating poor running conditions. Therefore, even in an automatic speed change mechanism having plural programs, the driver has to manipulate the conversion switches in response to the current situation and such manipulations of the switches are troublesome tasks. But in trucks and buses in which the variations of the load are very large, such switch-overs of the speed change programs are very effective in improving the fuel economy.